Sacrificial zinc anode



United States Patent 3,226,314 SACRIFICIAL ZINC ANODE John R. Wellington and William L. MacLean Phillips,

Trail, British Columbia, Canada, assignors to The Consolidated Mining and smelting Company of Canada Limited, Montreal, Quebec, Canada, a corporation of Canada No Drawing. Filed Aug. 9, 1962, Ser. No. 215,795 8 Claims. (Cl. 204-197) This invention relates to a new and useful sacrificial or consumable anode of the type used for inhibiting corrosion of metal structures by galvanic action. It is particularly directed to providing a sacrificial anode which can be used safely for the protection against corrosion by galvanic action of metal tanks, ships hulls, and the like which may contain explosive gas mixtures.

Sacrificial or consumable anodes are well known and are Widely used to protect metal structures, such as heat exchangers, pipe lines, storage tanks, ships hulls and the like, from corrosion by galvanic action. Such anodes usually comprise an exposed zinc, aluminum or magnesium body and a metal core. The exposed zinc, aluminum or magnesium body constitutes the corrodible or consumable portion of the anode. The core material, which must be cathodic to the anode metal, serves to reinforce or strengthen the anode body, to provide an electrical connection to the metallic structure to be protected and, also, to provide a strong mechanical connection for supporting the anode body and attaching it to the metal structure. The core metal thus must be strong and durable and there should be no corrosion of the connections between the core and the metal structure while metal of the anode body is still available for galvanic action. Also, it is desirable that the core be embedded in the anode metal in liquid-tight engagement to prevent the penetration of liquid, for example, sea water, along the junction between core and anode metal. The corrosion products that would result from such penetration would electrically insulate the core from the anode metal and would thereby make the anode ineffective.

The use of magnesium, aluminum and their alloys in iron or steel tanks containing combustible or explosive atmospher'es is a known hazard owing to the possibility of forming sparks when an article made of magnesium or aluminum or their alloys strikes a rusty iron surface. This objection does not apply to zinc. In view of this hazard, sacrificial anodes of magnesium or aluminum, and sacrificial anodes of zinc with aluminum cores, should not be installed to protect metal structures used to contain highly combustible materials, such as gasoline and oil, which can lead to the formation of explosive gas mixtures. Anoders are required to protect such structur'es, e.g., oil tankers, which may at times contain sea water as ballast.

Iron, steel and aluminum are usually used as core material for anodes. These metals have the desired properties of mechanical strength, durability and electrical conductivity, but aluminum can cause the sparking hazard, and iron and steel are not always easily fabricated to produce desired shapes, e.g., by extrusion of composite billets of anode metal and core material to form long, narrow anodes.

We have found that We can produce a very satisfactory and safe sacrificial anode by combining a zinc anode body with a zinc alloy core cathodic thereto. We have also found that the zinc alloy core should be comprised essentially of zinc which contains a minor amount of at least one member of the group consisting of iron, nickel and cobalt. This anode does not cause the sparking hazard that is caused by anodes which include magnesium ice or aluminum as body or core material. Also, the zinc alloy core of this invention is cathodic to the Zinc anode body and does not readily corrode in saltwater.

The maximum amount of alloying element or elements present in the alloy is determined by practical and economic factors. It is difiicult to make alloys containing more than about 4% by weight of the alloying element or elements. Also, it is difficult to extrude alloys containing more than about 2%, by weight, of the alloying element or elements. Accordingly, cast anodes would not include cores containing more than about 4%, by weight, of iron or nickel or cobalt or combinations thereof, and extruded anodes would not include cores containing more than about 2%, by weight, of iron or nickel or cobalt or combinations thereof.

The minimum amount of alloying element or elements required in the alloy core material is determined by the degree to which the core material is cathodic to the anode metal. We have found that the minimum amount should not be less than about 0.03%, by weight, of iron, nickel or cobalt or combinations thereof.

The best results are obtained with amounts of alloying elements ranging from a total of 0.3% to 1.5% by weight.

We have produced very satisfactory zinc anodes with zinc alloy cores containing as alloying ingredient each of the elements iron, nickel and cobalt, and the various combinations thereof. For example, we have made binary zinc alloy cores from zinc alloys which contained 0.1%, 0.3% and 0.5% by weight of iron; from zinc alloys which contained 0.1%, 0.3% and 0.5% by Weight of nickel; and from zinc alloys which contained 0.1%, 0.3% and 0.5% by weight of cobalt. Also, satisfactory ternary zinc alloy cores have been produced which contained about 0.15% by weight of iron and about 0.15% by weight of nickel; about 0.15% by weight of iron and about 0.15% by weight of cobalt; and about 0.15% by weight of nickel and about 0.15% by weight of cobalt. A ternary alloy which has been found to be very satisfactory contains, by w'eight, about 0.2% iron, about 0.2% nickel, and the balance special high grade zinc. The alloys of this invention containing cobalt appear to become passive slightly more readily than the alloys of this invention which do not contain cobalt. However, the difference in results is small and would not normally warrant the added expense of using the cobalt alloys as the core material.

The metal of the anode body of the anode of the present invention can be the same as that normally used with commercial zinc anodes, e.g., high purity zinc containing less than 0.0014% iron; special high grade zinc containing 0.1% to 0.5% aluminum and 0.03% to 0.1% cadmium; and special high grade zinc containing about 0.4% aluminum.

The zinc alloy for the core can be prepared by dissolving the required amount of iron, nickel or cobalt in a bath of molten zinc held at a temperature of about 1400 to 1500 F. At this temperature, a holding time of about 6 hours with frequent stirring is necessary to obtain complete solution. The holding time can be shortened by the use of higher melting temperatures and finely divided iron, nickel or cobalt.

The anode can be produced by various methods depending on the desired product and available facilities. For example, the core alloy can be cast into the desired shape or cast and then extruded. The anode metal can be cast around the core material to form a composite billet for subsequent extrusion, or to form the final anode shape.

In casting the zinc anode metal around the zinc alloy core, it is important to ensure that the anode metal does not become contaminated with the core alloy. A suitable procedure is to maintain the molten zinc anode metal at a temperature only slightly above its melting point and to pour the molten metal into the mould containing the zinc alloy core in such a manner that the molten stream of metal does not impinge on the alloy core, for example, the molten metal can be directed against the inner surface of the mould.

A satisfactory procedure for making anodes of the present invention includes the step of casting the alloy core material in the form of cylindrical billets about 18 inches long and 4.5 inches in diameter, and then to extrude these billets to form core material in the shape of cylindrical rods about two inches in diameter. Anode metal can be cast around a core rod to form a symmetrical composite billet about 18 inches long and 4.5 inches in diameter. Extrusion of this composite billet through a round one inch die opening, e.g., as described in US. Patent No. 3,024,183, will produce cylindrical anode stock about 1 inch in diameter with an alloy core about /3 inch in diameter.

Extrusion of a composite billet through a suitable two hole die, as described in United States Patent No. 3,152,- 059, can be used to produce anode strips with core exposed along one surface of each strip.

The sacrificial or consumable anode of this invention has many important advantages. It is easily produced, and the zinc body is readily corrodible by galvanic action and provides complete protection to the metallic structure to which the anode is attached as long as any of the zinc body remains. The zinc alloy core is embedded in the anode metal in liquid-tight engagement along the length of the anode to form, in effect, a unitary structure. The zinc alloy core possesses the necessary mechanical strength for secure attachment to the metal structure to be protected; it provides a satisfactory electrical connection with the metal structure; there is little or no corrosion of the core as long as metal of the anode body is available for galvanic action; and there is no danger of sparks if the anode happens to drop or strike against iron or steel. Also, the anode can be produced as a casting, or as an extruded rod or ribbon adapted to fit to the shape of the structure the anode is designed to protect.

In tests conducted on the anodes of this invention in which the zinc anode metal was dissolved galvanically using a steel cathode in a 3% sodium chloride solution it was established that the zinc alloy core is cathodic to the zinc anode body; that the alloy core materials have no adverse effect on the action of the anode; and that the alloy core material remains undissolved until after the anode metal has been dissolved.

It will be understood, of course, that modifications can be made in the preferred embodiments of the invention described herein without departing from the scope of the invention defined by the appended claims.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. A sacrificial anode which comprises a zinc anode body bonded to a zinc alloy core cathodic to and embedded in the zinc anode body, said zinc alloy core consisting of zinc and 0.03% to 4% by weight of one member selected from the group consisting of iron, nickel and cobalt.

2. A sacrificial anode which comprises a zinc anode body bonded to a zinc alloy core, said zinc alloy core consisting of zinc and 0.03% to 4% by weight of any two members selected from the group consisting of iron, nickel and cobalt.

3. A sacrificial anode which comprises a zinc anode body bonded to a zinc alloy core cathodic to and embedded in the zinc anode body, said zinc alloy core consisting of zinc and 0.03% to 2% by weight of one member selected from the group consisting of iron, nickel and cobalt.

4. A sacrificial anode which comprises a zinc anode body bonded to a zinc alloy core, said zinc alloy core consisting of zinc and 0.03% to 2% by weight of any two members selected from the group consisting of iron, nickel and cobalt.

5. A sacrificial anode which comprises a zinc anode body bonded to a zinc alloy core cathodic to and embedded in the zinc anode body, said zinc alloy core con sisting of zinc and 0.03% to 4% by weight of one member selected from the group consisting of iron, nickel and cobalt, and said core being exposed along the length of one surface of the anode.

6. A sacrificial anode which comprises a zinc anode body bonded to a zinc alloy core, said zinc alloy core consisting of zinc and 0.03% to 4% by weight of any two members selected from the group consisting of iron, nickel and cobalt, and said core being exposed along the length of one surface of the anode.

7. A sacrificial anode which comprises a zinc anode body bonded to a zinc alloy core cathodic to and embedded in the zinc anode body, said zinc alloy core consisting of zinc and 0.03% to 2% by weight of one member selected from the group consisting of iron, nickel and cobalt and said core being exposed along the length of one surface of the anode.

8. A sacrificial anode which comprises a zinc anode body bonded to a zinc alloy core, said zinc alloy core consisting of zinc and 0.03% to 2% by weight of any two members selected from the group consisting of iron, nickel and cobalt, and said core being exposed along the length of one surface of the anode.

References Cited by the Examiner UNITED STATES PATENTS 1,506,772 9/1924 Pack 178.1 1,928,053 9/1933 Freeman 75178.1 2,172,933 9/1939 Daesen et al. 75-178.1 2,478,479 8/1949 Grebe et al. 204-197 2,645,612 7/1953 Taylor 204-197 2,772,231 11/1956 Waite et al. 204-197 3,024,183 3/1962 MacEwan 204197 FOREIGN PATENTS 904,543 3/ 1945 France.

OTHER REFERENCES 7 Metals and Alloys, volume 20, October 1944, page 1068.

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, WINSTON A. DOUGLAS,

Examiners. 

1. A SACRIFICIAL ANODE WHICH COMPRISES A ZINC ANODE BODY BONDED TO A ZINC ALLOY CORE CATHODIC TO AND EMBEDDED IN THE ZINC ANODE BODY, SAID ZINC ALLOY CORE CONSISTING OF ZINC AND 0.03% TO 4% BY WEIGHT OF ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF IRON, NICKEL AND COBALT. 